Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss.
Optical networks often employ wavelength division multiplexing (WDM) to increase transmission capacity. In a WDM network, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is increased as a multiple of the number of wavelengths, or channels, in each fiber.
The maximum distance that a signal can be transmitted in a WDM or other optical network without amplification is limited by absorption, scattering and other loss associated with the optical fiber. To transmit signals over long distances, optical networks typically include a number of discrete amplifiers spaced along each fiber route. The discrete amplifiers boost received signals to compensate for transmission losses in the fiber.
Signals may also be boosted in the fiber using Raman effect amplification. In the Raman effect, optical signals traveling in the fiber are amplified by the presence of a lower wavelength pump light traveling in the same fiber. The pump light may travel forward with the signal or backwards in reverse of the signal. Because forward pumping amplification causes cross talk between channels in WDM systems due to cross gain modulation (XGM), reverse pumping amplification is typically used in connection with WDM and other multichannel systems. This limits the use of Raman effect amplification in multichannel systems to unidirectional transmission fibers and prevents bi-directional distributed amplification.